The Foundations of Chemistry

7-7 Limitations of the Octet Rule for Lewis Formulas 291

LIMITATIONS OF THE OCTET RULE FOR

LEWIS FORMULAS

Recall that representative elements achieve noble gas electron configurations in mostof
their compounds. But when the octet rule is not applicable, the relationship SNA
is not valid without modification. The following are general cases for which the proce-
dure in Section 7-5 must be modified—that is, there are four types of limitations of the
octet rule.

A. Most covalent compounds of beryllium, Be. Because Be contains only two valence

shell electrons, it usually forms only two covalent bonds when it bonds to two other

atoms. We therefore use four electronsas the number neededby Be in step 2, Section

7-5. In steps 3 and 4 we use only two pairs of electrons for Be.

B. Most covalent compounds of the Group IIIA elements, especially boron, B. The

IIIA elements contain only three valence shell electrons, so they often form three

covalent bonds when they bond to three other atoms. We therefore use six electrons

as the number neededby the IIIA elements in step 2; and in steps 3 and 4 we use

only three pairs of electrons for the IIIA elements.

C.Compounds or ions containing an odd number of electrons. Examples are NO,

with 11 valence shell electrons, and NO 2 , with 17 valence shell electrons.

D.Compounds or ions in which the central element needs a share in more than eight

valence shell electrons to hold all the available electrons, A. Extra rules are added

to steps 2 and 4 of the procedure in Section 7-5 when this is encountered.

Step 2a: If S, the number of electrons shared, is less than the number needed to

bond all atoms to the central atom, then Sis increased to the number of

electrons needed.

Step 4a: If Smust be increased in step 2a, then the octets of all the atoms might

be satisfied before all of the electrons (A) have been added. Place the

extra electrons on the central element.

Many species that violate the octet rule are quite reactive. For instance, compounds
containing atoms with only four valence shell electrons (limitation type A above) or six
valence shell electrons (limitation type B above) frequently react with other species that
supply electron pairs. Compounds such as these that accept a share in a pair of electrons
are called Lewis acids;a Lewis baseis a species that makes available a share in a pair of elec-
trons. (This kind of behavior will be discussed in detail in Section 10-10.) Molecules with
an odd number of electrons often dimerize(combine in pairs) to give products that
do satisfy the octet rule. Examples are the dimerization of NO to form N 2 O 2 (Section
24-15) and NO 2 to form N 2 O 4 (Section 24-15). Examples 7-5 through 7-9 illustrate some
limitations and show how such Lewis formulas are constructed.

EXAMPLE 7-5 Limitations of the Octet Rule

Write the Lewis formula for gaseous beryllium chloride, BeCl 2 , a covalent compound.

Plan

This is an example of limitation type A. So, as we follow the steps in writing the Lewis formula,
we must remember to use four electronsas the number neededby Be in step 2. Steps 3 and 4
should show only two pairs of electrons for Be.

7-7

Lewis formulas are not normally

written for compounds containing

d- and f-transition metals. The d- and

f-transition metals utilize dor forbitals

(or both) in bonding as well as sand p

orbitals. Thus, they can accommodate

more than eight valence electrons.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 9.8, Free Radicals—Exceptions

to the Octet Rule.